Microscope with light source

ABSTRACT

A microscope with incident light input coupling, wherein the light provided for the incident illumination is directed onto the partially reflecting layer of a beam splitter cube and is directed from there through the objective onto the specimen, while the light reflected and/or emitted by the specimen travels back to the partially reflecting layer and passes through the latter into the imaging beam path. In a microscope of this type, the beam splitter cube is provided with a negative spherical curvature at its outer surface facing the objective. Further, instead of the conventional tube lens, there is a combination formed of a converging lens and a diverging lens, wherein the surface curvatures of the converging lens and the diverging lens and the negative spherical curvature effected at the beam splitter cube are adapted to one another in such a way that the back-reflections of the incident illumination in the intermediate image plane are limited to a minimum.

BACKGROUND OF THE INVENTION

a) Field of the Invention

The invention is directed to a microscope with incident light inputcoupling, wherein the light provided for the incident illumination isdirected to the partially reflecting layer of a beam splitter cube andis directed from there through the microscope objective onto thespecimen, while the light reflected and/or emitted by the specimentravels back to the partially reflecting layer and passes through thelatter into the imaging beam path.

b) Description of the Related Art

Much more than with transmitted light illumination, the quality ofimages generated with incident light is characterized by controlling thereflections of optical elements in the beam path that is utilized forboth illumination and imaging;

these reflections are sometimes extremely troublesome. Therefore, asregards optical design, it must be ensured consistently for each elementand each surface in the beam path that the back-reflection of the latteris damped, for instance, by anti-reflection coating of the surface forthe relevant spectral region and/or that the position of the ghost imageor reflection image of the illumination source (of every surface) islocated far away from image planes.

The known steps for eliminating surface reflections on the objectives ofthe primary tube are directed, above all, to broad-band anti-reflectioncoating of all lens surfaces of a mounted objective system. In moderndesign of high-quality objectives for incident light applications, thecurvatures of the individual lenses are generally adapted to present-dayrequirements for low reflection in the intermediate image, aside fromthe typical requirements for the imaging system for achieving adiffraction-limited transmission of the object plane in the image plane.With most microscope manufacturers, the microscope objectives arecorrected to infinity on the image side, i.e., a plane wave surface isgenerated which first generates a real imaging of the object in theintermediate image plane through the tube lens.

The typical procedure for coupling in the illumination source inincident light applications is neutral, dichroic or polarization-opticalinput reflection of a parallelized light bundle of the illuminationsource by means of a plane input-coupling element such as aplane-parallel coated splitter plate or square splitter cube.Accordingly, light is usually coupled into the parallelized infinitebeam path between the objective and tube lens. FIG. 1 shows theconventional arrangement for incident illumination in the parallel beampath between the tube lens and objective.

Problems arise with typical incident light input-coupling in thenon-parallel, finite beam path of a microscope. When coupling in via aninclined plane plate, aberrations are generated, so that this type ofinput-reflection of light is not suitable for high-grade opticalsystems. On the other hand, coupling in via a beam splitter cube betweenthe image plane and tube lens generally generates a relatively strongback-reflection at the lower plane surface of the beam splitter cube.All of the plane surfaces and many optical surfaces with a relativelyslight curvature, e.g., the surfaces of the tube lens which usually hasa quite long focal length, generate troublesome reflections withincident illumination which reduce the image contrast and imagingquality.

However, particularly with respect to modern microscopy methods,coupling in of incident light between the tube lens and intermediateimage plane offers specific advantages, e.g., with respect to theutilization of space in the primary tube of the microscope, so thatthere is a particular need to solve the problems mentioned above.

An arrangement for low-reflection input-coupling of the illuminationsource in a parallel confocal incident light microscope is described inDE 19511937 C2. In this case, the neutral, dichroic orpolarization-optical input-reflection of the illumination source iscarried out above the intermediate image plane and a splitter body whichis cut in a rhombic shape is used to prevent troublesome reflections ofthe plane input-coupling element. In order to reduce the error effect ofthe rhombus during image formation in the imaging beam path,compensating wedges are required in front of and behind the intermediateimage plane. The Nipkow disk, the confocal element in the illuminationand imaging beam path, is located in the intermediate image plane and iscoated with a broad-band anti-reflection coating, also to preventtroublesome reflections at this element, and arranged in the beam pathat a slight inclination.

Another solution for eliminating the troublesome influence of surfacesof plane splitter elements is described in DE 4446134 A1, in which veryweak reflections in the fundus oculi must be detected in aninterferometric arrangement for measuring the length of the eye. Forthis purpose, either rhomboidal plane splitter elements or bodies whichare slightly skewed but planar are used as splitter elements forcoupling in the illumination (semiconductor laser) and for generating areference beam path of the interferometric measurement principle. Theseelements are difficult to handle during production as well as foradjustment in the overall arrangement.

In the German Patent DE 19714221 (in contrast to the above-cited DE19511937 C2), two confocal disks with pinhole arrays are used forpreventing strong interfering reflections proceeding from theillumination so that the primary illumination reflections are not evenallowed to reach the imaging arm. This is effected at the expense of acomplicated adjustment for producing the conjugation of the illuminationarray to the pinhole array.

OBJECT AND SUMMARY OF THE INVENTION

Based on this prior art, it is the primary object of the invention toimprove the image quality and therefore the utility value properties ina microscope of the type described above by reducing the influence offalse light.

This object of the invention is met in that the beam splitter cube has anegative spherical curvature at its outer surface facing the objective.

Due to the spherical surface curvature carried out at the beam splittercube, the latter acts like a piano-concave lens, so that theback-reflections of the incident illumination in the intermediate imageplane are reduced because the light reflections of this surface aresharply reduced by the concave surface produced on the splitter.

This effect is reinforced in an arrangement of the invention byproviding a combination of a diverging lens and a converging lens,wherein the surface curvatures of the diverging lens and converging lensand the negative spherical curvature effected at the beam splitter cubeare adapted to one another in such a way that the back-reflections ofthe incident illumination in the intermediate image plane are limited toa minimum. This is achieved through a deliberate and defined increase inthe radius curvatures of the two lenses and of the curved surface at thebeam splitter cube. In so doing, the splitter body acts optically as athick piano-concave lens (negative lens). The diverging lens, theconverging lens and the negative spherical curvature effected at thebeam splitter cube replace the tube lens that is conventionallyprovided.

According to the invention, it is provided in a further constructionalvariant that the diverging lens—for purposes of achieving sharply curvedradii—is formed as a curved negative lens (biconcave lens) and theconverging lens is formed as a biconvex lens, both lenses workingtogether with the polished concave surface at the beam splitter cube aswas described above. As a result, the negative optical action of thebeam splitter and light bundle expansion by means of the above-mentioneddiverging lens with an increased positive refractive power acts inopposition to the converging lens in a compensating manner.

This increase in the refractive power of the converging lens results ingreater surface curvatures at the relevant optical elements andaccordingly in a substantially stronger dispersive behavior with respectto the resulting back-reflections.

In this way, the disadvantages of incident light coupling areextensively eliminated by two relatively sharply curved diverging lensesand a relatively sharply curved converging lens which cooperate as asubstitute tube lens. While coupling in of light between the tube lensand microscope intermediate image by conventional means (standardsplitter and tube lens) leads to unacceptable false light problems, thedescribed arrangement offers the possibility of a substantially improvedsuppression of interfering reflections.

Further, this arrangement opens up possibilities for expanding thefunctionality of the optical primary tube. Accordingly, it is providedaccording to the invention that both the converging lens and thediverging lens are arranged so as to be displaceable individually orjointly in the direction of the image plane, wherein the distancebetween the two lenses and/or between the two lenses and the image planecan be changed as a result of the displacement, which results in achange in the resulting focal length.

Therefore, zoom effects can be achieved within a small area by varyingthe distance, resulting in advantages for applications in which it isnecessary to calibrate the magnification in the microscope primary tubewith exactness. As is well known, the magnification scale of the primarytube in its entirety is subject to certain fluctuations because oftypical manufacture-related deviations of the magnification scale ofobjectives of the same type and of the focal length of standard tubelenses.

To enable exact calibration of the primary tube in scale, a zoomcharacteristic of the tube system is desirable in order that themagnification scale of the primary tube in its entirety can becalibrated by varying the focal length. For this purpose, a known scalecan be imaged via the primary tube on a camera and the magnificationscale can be measured. For calibration, the tube system is tuned, i.e.,its total focal length is adjusted to a suitable measure by changing theair clearance.

By changing the distance between the diverging lens and the converginglens or by displacing both of these lenses jointly, it is possible tovary the total focal length (although within tight limits), and,therefore, the linear magnification or imaging scale, without asubstantial decline in imaging quality. The variation of the total focallength is given by the following equation:${f_{a} - f_{e}} = \frac{d_{a} - d_{e}}{\left( {f_{2} + f_{1} - d_{a}} \right)\left( {f_{2} + f_{1} - d_{e}} \right)}$

The variation of the intersection length or back focus is given by thefollowing equation:${s_{a}^{\prime} - s_{e}^{\prime}} = {\frac{\left( {f_{2} - d_{a}} \right) \times f_{1}}{\left( {f_{1} + f_{2} - d_{a}} \right)} - \frac{\left( {f_{2} - d_{e}} \right) \times f_{1}}{\left( {f_{1} + f_{2} - d_{e}} \right)}}$

where f_(a) is the total focal length before the adjustment, f_(e) isthe total focal length after the adjustment, f₁ is the focal length ofthe converging lens, f₂ is the focal length of the diverging lens, d_(a)is the distance between the converging lens and diverging lens beforethe adjustment, d_(e) is the distance between the converging lens anddiverging lens after the adjustment, s_(a)′ is the back focus beforeadjustment and s_(e)′ is the back focus after the adjustment (see FIG.4).

For example, this offers the possibility of an important application forachieving an exact optical imaging scale in automatic inspectionmachines in which the cell-to-cell comparison is carried out principallyin periodic storage memory modules.

With regard to the optical inspection of periodic structures insemiconductor components, high productivities are achieved withinspection since the same optical structures and therefore the sameelectronic signals should occur within a receiver field. The electronicsignals can be inspected immediately for absolute identity by fasthardware without buffering. However, this procedure is only reliablewhen no aliasing effects occur, i.e., when the optical length of afreely selectable period structure gives a whole pixel quantity in exactsubpixels. This is possible optically only when the imaging scale of theperiod length which generally differs in circuit structures of differentdegrees of integration, with different manufacturers, techniques or thelike is adjusted very exactly and dynamically to the cameracharacteristics (pixel size).

In this context, another construction of the invention provides that theconverging lens and/or the diverging lens are/is coupled with amotor-driven actuating member and this actuating member is connectedwith a device for issuing actuating commands.

A sensitive adjustment of the zoom characteristics in the parallelconfocal primary tube of an inspection device is made possible by meansof suitably designed motor-driven actuating members. The dynamicadjustment of the imaging scale in a secondary imaging specifically to areceiver circuit with a confocal character entails very greatdifficulties.

The invention will be described more fully in the following withreference to a embodiment example.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawing:

FIG. 1 the principle of incident light input-coupling according to theprior art;

FIG. 2 the principle of incident light input-coupling according to theinvention;

FIG. 3 the application of incident light input-coupling according to theinvention in connection with a field lens;

FIG. 4 a view illustrating the change of the focusing state; and

FIG. 5 an arrangement with changeable focusing state.

DESCRIPTION OF THE EMBODIMENTS

In contrast to the prior art which is shown in FIG. 1 and which wasalready mentioned in the introductory part of the specification, theincident light input-coupling for a microscope corresponding to theinvention is shown in FIG. 2. In this case, the illumination beam path 1provided for incident illumination is directed onto the partiallyreflecting layer 2 of a beam splitter cube 3. The inclined position ofthe partially reflecting layer 2 causes the deflection of theillumination beam path 1 in the direction of a specimen 4. The lightreflected and/or emitted by the specimen 4 travels back to the partiallyreflecting layer 2 and passes through the latter into the imaging beampath 5.

According to the invention, the beam splitter cube 3 has a negativespherical curvature at its outer surface 6 which faces the specimen 4and, therefore, the imaging objective 17.

Further, a converging lens 7 and a diverging lens 8 are arranged in thebeam path between the beam splitter cube 3 and the imaging objective 17.The surface curvatures of the converging lens 7, the diverging lens 8and the curvature of the outer surface 6 are adapted to one another insuch a way that, through their cooperation, the interfering secondaryillumination reflections are suppressed to a minimum. This is achievedin that the beam splitter cube 3 which acts as a thick plano-concavelens during the passage of the imaging beam path due to the curvature ofthe outer surface 6 produces a negative optical effect, as does thelight bundle expansion of the diverging lens 8.

According to the invention, the negative optical action of the beamsplitter 3 and diverging lens 8 acts in opposition to the converginglens 7 with a relatively high refractive power. The increase in therefractive power leads in the converging lens to larger surfacecurvatures and accordingly to the desired increased dispersive behaviorof the back-reflections. Accordingly, the disadvantages of the prior artwith respect to incident light input-coupling between the tube lens andimage plane are eliminated to a great extent.

FIG. 3 shows the construction of the arrangement according to theinvention with reference to an example with a Nipkow disk as a confocalelement in the illumination and imaging beam path in which thenegatively curved outer surface 6 at the beam splitter cube 3 cooperateswith a field lens 9 as a substitute field lens. In this case, also, aconverging lens 7 and a diverging lens 8 are provided, wherein theNipkow disk 10 is arranged so as to be inclined in the intermediateimage plane in the beam path between the beam splitter cube 3 and theconverging lens 7, and wherein a pair of wedges 11, 12 is associatedwith the Nipkow disk 10.

FIG. 4 shows, by way of example, the focusing state with reference tothe diverging lens 8 and the converging lens 7. At FIGS. 4a and 4 b, theconverging lens 7 has focal length f₁, the diverging lens 8 has focallength f₂. By varying the distance between the converging lens 7 and thediverging lens 8 or simultaneously displacing the lens group comprisingthe converging lens 7 and diverging lens 8, it is possible to vary thetotal focal length within close limits and accordingly to change theimaging scale without substantially worsening the imaging quality.

The variation of the total focal length is given by Δf=f_(a)−f_(e) andthe variation in the back focus is given by Δs=s_(a)−s_(e) correspondingto the equations indicated in the preceding description in which theinfluence of the focal lengths f₁, f₂ and distance d in the variation ofthe total focal length and back focus is thoroughly described.

FIG. 5 shows an application example for varying the focal length bychanging the distance between the converging lens 7 and diverging lens 8or the disstance between the converging lens 7 and diverging lens 8 andthe beam splitter cube 3. In this case, the converging lens 7 is coupledwith an actuating member 13 and the diverging lens 8 is coupled with anactuating member 14. Both actuating members 13 and 14 are connected witha control unit 15 which has an input module 16 for entering actuatingcommands.

When actuating commands are entered via the input module 16 (manually orautomatically), a displacement of the converging lens 7 and/or diverginglens 8 is caused via the control unit 15 and actuating members 13 and14; in so doing, the converging lens 7 and/or diverging lens 8 move on aguide in the direction of the beam path. When this happens, the totalfocal length and the back focus change corresponding to the equationgiven above. In this way, this arrangement opens up possibilities forexpanding the functionality of the optical primary tube for purposes ofan exact calibration of magnification.

While the foregoing description and drawings represent the presentinvention, it will be obvious to those skilled in the art that variouschanges nay be made therein without departing from the true spirit andscope of the present invention.

REFERENCE NUMBERS

1 illumination beam path

2 partially reflecting layer

3 beam splitter cube

4 specimen

5 imaging beam path

6 negatively curved outer surface

7 converging lens

8 diverging lens

9 field lens

11, 12 wedge pair

13, 14 actuating member

15 control unit

16 input module

17 imaging objective

What is claimed is:
 1. A microscope with incident light input-couplingcomprising: a beam splitter cube having a partially reflecting layer; animaging objective; light provided for incident light illumination beingdirected onto a partially reflecting layer of said beam splitter cubeand being directed from there by way of said imaging objective onto aspecimen; light emanating from the specimen traveling back to thepartially reflecting layer and passing through said layer in an imagingbeam path; and said beam splitter cube having a negative sphericalcurvature at its outer surface facing the imaging objective.
 2. Themicroscope according to claim 1, said microscope including a tube lenssystem, wherein a converging lens and a diverging lens are provided insaid tube lens system, wherein surface curvatures of the diverging lens,converging lens and beam splitter cube, which acts as a piano-concavelens due to the surface curvature, are adapted to one another in such away that secondary illumination reflections are limited to a minimum. 3.The microscope according to claim 2, wherein the converging lens isformed as a biconvex lens and the diverging lens is formed as abiconcave lens.
 4. The microscope according to claim 2, wherein both theconverging lens and the diverging lens are arranged so as to bedisplaceable individually or jointly toward or away from the imageplane, wherein the distance between the two lenses and/or between thetwo lenses and the image plane can be changed as a result of thedisplacement, which results in a change in a focal length.
 5. Themicroscope according to claim 4, wherein the converging lens and thediverging lens are coupled with motor-driven actuating members and areconnected via the latter with a control unit and with an input modulefor entering actuating commands.
 6. The microscope according to claim 1,wherein a confocal element is arranged in an intermediate image plane,wherein a diverging lens and a converging lens are provided in a tubelens system and a field lens is provided in an imaging beam path,wherein the surface curvatures of the converging lens, the diverginglens, the beam splitter cube, which acts as a plano-concave lens due tothe surface curvature, and the field lens are adapted to one another insuch a way that secondary illumination reflections are limited to aminimum.